Reveal UN terribly evil (6)
I believe the answer is:
'reveal' is the definition.
(I know that reveal can be written as unveil)
'un terribly evil' is the wordplay.
'terribly' is an anagram indicator.
'evil' anagrammed gives 'veil'.
(Other definitions for unveil that I've seen before include "Expose by removing covering" , "Remove covering, expose" , "Introduce publicly for the first time" , "Reveal by removing a curtain" , "Remove headdress" .)